[RFC,1/2] kunit: Expose 'static stub' API to redirect functions

Commit Message

David Gow March 18, 2022, 2:13 a.m. UTC

Add a simple way of redirecting calls to functions by including a
special prologue in the "real" function which checks to see if the
replacement function should be called (and, if so, calls it).

To redirect calls to a function, make the first (non-declaration) line
of the function:

	KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);

(This will compile away to nothing if KUnit is not enabled, otherwise it
will check if a redirection is active, call the replacement function,
and return.)

Calls to the real function can be redirected to a replacement using:

	kunit_activate_static_stub(test, real_fn, replacement_fn);

The redirection will only affect calls made from within the kthread of
the current test, and will be automatically disabled when the test
completes. It can also be manually disabled with
kunit_deactivate_static_stub().

The 'example' KUnit test suite has a more complete example.

This is intended to be an alternative to ftrace-based stubbing (see the
next patch), with different tradeoffs.

In particular:
- 'static stubs' work on all architectures, and don't have any
  dependencies.
  - There's no separate Kconfig option to enable them, they always
    exist.
- They must be explicitly opted-into by the function being replaced
  (which can be good or bad depending on your POV)
- They have a greater performance penalty when not in active use (as
  every call to the real function must check to see if the stub is
  enabled)
- They could more easily be extended to pass additional context to
  replacement functions.

Co-developed-by: Daniel Latypov <[e-mail]>
Signed-off-by: Daniel Latypov <[e-mail]>
Signed-off-by: David Gow <[e-mail]>
---
 include/kunit/static_stub.h    | 106 ++++++++++++++++++++++++++++
 lib/kunit/Makefile             |   1 +
 lib/kunit/kunit-example-test.c |  39 ++++++++++
 lib/kunit/static_stub.c        | 125 +++++++++++++++++++++++++++++++++
 4 files changed, 271 insertions(+)
 create mode 100644 include/kunit/static_stub.h
 create mode 100644 lib/kunit/static_stub.c

Comments

Brendan Higgins April 4, 2022, 8:16 p.m. UTC | #1

+Steve Muckle

On Thu, Mar 17, 2022 at 10:13 PM David Gow <[e-mail]> wrote:
>
> Add a simple way of redirecting calls to functions by including a
> special prologue in the "real" function which checks to see if the
> replacement function should be called (and, if so, calls it).
>
> To redirect calls to a function, make the first (non-declaration) line
> of the function:
>
>         KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);
>
> (This will compile away to nothing if KUnit is not enabled, otherwise it
> will check if a redirection is active, call the replacement function,
> and return.)
>
> Calls to the real function can be redirected to a replacement using:
>
>         kunit_activate_static_stub(test, real_fn, replacement_fn);
>
> The redirection will only affect calls made from within the kthread of
> the current test, and will be automatically disabled when the test
> completes. It can also be manually disabled with
> kunit_deactivate_static_stub().
>
> The 'example' KUnit test suite has a more complete example.
>
> This is intended to be an alternative to ftrace-based stubbing (see the
> next patch), with different tradeoffs.
>
> In particular:
> - 'static stubs' work on all architectures, and don't have any
>   dependencies.
>   - There's no separate Kconfig option to enable them, they always
>     exist.
> - They must be explicitly opted-into by the function being replaced
>   (which can be good or bad depending on your POV)
> - They have a greater performance penalty when not in active use (as
>   every call to the real function must check to see if the stub is
>   enabled)
> - They could more easily be extended to pass additional context to
>   replacement functions.
>
> Co-developed-by: Daniel Latypov <[e-mail]>
> Signed-off-by: Daniel Latypov <[e-mail]>
> Signed-off-by: David Gow <[e-mail]>
> ---
>  include/kunit/static_stub.h    | 106 ++++++++++++++++++++++++++++
>  lib/kunit/Makefile             |   1 +
>  lib/kunit/kunit-example-test.c |  39 ++++++++++
>  lib/kunit/static_stub.c        | 125 +++++++++++++++++++++++++++++++++
>  4 files changed, 271 insertions(+)
>  create mode 100644 include/kunit/static_stub.h
>  create mode 100644 lib/kunit/static_stub.c
>
> diff --git a/include/kunit/static_stub.h b/include/kunit/static_stub.h
> new file mode 100644
> index 000000000000..020e0e9110f2
> --- /dev/null
> +++ b/include/kunit/static_stub.h
> @@ -0,0 +1,106 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * KUnit function redirection (static stubbing) API.
> + *
> + * Copyright (C) 2022, Google LLC.
> + * Author: David Gow <[e-mail]>
> + */
> +#ifndef _KUNIT_STATIC_STUB_H
> +#define _KUNIT_STATIC_STUB_H
> +
> +#if !IS_REACHABLE(CONFIG_KUNIT)
> +
> +/* If CONFIG_KUNIT is not enabled, these stubs quietly disappear. */
> +#define KUNIT_TRIGGER_STATIC_STUB(real_fn_name, args...) do {} while (0)
> +
> +#else
> +
> +#include <kunit/test.h>
> +
> +#include <linux/compiler.h> /* for {un,}likely() */
> +#include <linux/sched.h> /* for task_struct */
> +
> +/* Returns the address of the replacement function. */
> +void *__kunit_get_static_stub_address(struct kunit *test, void *real_fn_addr);
> +
> +/**
> + * KUNIT_STATIC_STUB_REDIRECT() - call a replacement 'static stub' if one exists
> + * @real_fn_name: The name of this function (as an identifier, not a string)
> + * @args: All of the arguments passed to this function
> + *
> + * This is a function prologue which is used to allow calls to the current
> + * function to be redirected by a KUnit test. KUnit tests can call
> + * kunit_activate_static_stub() to pass a replacement function in. The
> + * replacement function will be called by KUNIT_TRIGGER_STATIC_STUB(), which
> + * will then return from the function. If the caller is not in a KUnit context,
> + * the function will continue execution as normal.
> + *
> + * Example:
> + *
> + * .. code-block:: c
> + *     int real_func(int n)
> + *     {
> + *             KUNIT_STATIC_STUB_REDIRECT(real_func, n);
> + *             return 0;
> + *     }
> + *
> + *     void replacement_func(int n)
> + *     {
> + *             return 42;
> + *     }
> + *
> + *     void example_test(struct kunit *test)
> + *     {
> + *             kunit_activate_static_stub(test, real_func, replacement_func);
> + *             KUNIT_EXPECT_EQ(test, real_func(1), 42);
> + *     }
> + *
> + */
> +#define KUNIT_STATIC_STUB_REDIRECT(real_fn_name, args...)              \
> +do {                                                                   \
> +       typeof(&real_fn_name) replacement;                              \
> +       struct kunit *current_test = current->kunit_test;               \
> +                                                                       \
> +       if (likely(!current_test))                                      \
> +               break;                                                  \
> +                                                                       \
> +       KUNIT_ASSERT_STREQ_MSG(current_test, __func__, #real_fn_name,   \
> +                              "Static stub function name mismatch");   \
> +                                                                       \
> +       replacement = __kunit_get_static_stub_address(current_test,     \
> +                                                       &real_fn_name); \
> +                                                                       \
> +       if (unlikely(replacement))                                      \
> +               return replacement(args);                               \
> +} while (0)
> +
> +/* Helper function for kunit_activate_static_stub(). The macro does
> + * typechecking, so use it instead.
> + */
> +void __kunit_activate_static_stub(struct kunit *test,
> +                                 void *real_fn_addr,
> +                                 void *replacement_addr);
> +
> +/**
> + * kunit_activate_static_stub() - replace a function using static stubs.
> + * @test: A pointer to the 'struct kunit' test context for the current test.
> + * @real_fn_addr: The address of the function to replace.
> + * @replacement_addr: The address of the function to replace it with.
> + *
> + * When activated, calls to real_fn_addr from within this test (even if called
> + * indirectly) will instead call replacement_addr. The function pointed to by
> + * real_fn_addr must begin with the static stub prologue in
> + * KUNIT_TRIGGER_STATIC_STUB() for this to work. real_fn_addr and
> + * replacement_addr must have the same type.
> + *
> + * The redirection can be disabled again with kunit_deactivate_static_stub().
> + */
> +#define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {  \
> +       typecheck(typeof(real_fn_addr), replacement_addr);                      \
> +       __kunit_activate_static_stub(test, real_fn_addr, replacement_addr);     \
> +} while (0)
> +
> +void kunit_deactivate_static_stub(struct kunit *test, void *real_fn_addr);
> +
> +#endif
> +#endif
> diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
> index c49f4ffb6273..f9e929700782 100644
> --- a/lib/kunit/Makefile
> +++ b/lib/kunit/Makefile
> @@ -1,6 +1,7 @@
>  obj-$(CONFIG_KUNIT) +=                 kunit.o
>
>  kunit-objs +=                          test.o \
> +                                       static_stub.o \
>                                         string-stream.o \
>                                         assert.o \
>                                         try-catch.o \
> diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c
> index 51099b0ca29c..670c21e74446 100644
> --- a/lib/kunit/kunit-example-test.c
> +++ b/lib/kunit/kunit-example-test.c
> @@ -7,6 +7,7 @@
>   */
>
>  #include <kunit/test.h>
> +#include <kunit/static_stub.h>
>
>  /*
>   * This is the most fundamental element of KUnit, the test case. A test case
> @@ -69,6 +70,43 @@ static void example_mark_skipped_test(struct kunit *test)
>         /* This line should run */
>         kunit_info(test, "You should see this line.");
>  }
> +
> +/* This is a function we'll replace with static stubs. */
> +static int add_one(int i)
> +{
> +       /* This will trigger the stub if active. */
> +       KUNIT_STATIC_STUB_REDIRECT(add_one, i);
> +
> +       return i + 1;
> +}
> +
> +/* This is used as a replacement for the above function. */
> +static int subtract_one(int i)
> +{
> +       /* We don't need to trigger the stub from the replacement. */
> +
> +       return i - 1;
> +}
> +
> +/*
> + * This test shows the use of static stubs.
> + */
> +static void example_static_stub_test(struct kunit *test)
> +{
> +       /* By default, function is not stubbed. */
> +       KUNIT_EXPECT_EQ(test, add_one(1), 2);
> +
> +       /* Replace add_one() with subtract_one(). */
> +       kunit_activate_static_stub(test, add_one, subtract_one);
> +
> +       /* add_one() is now replaced. */
> +       KUNIT_EXPECT_EQ(test, add_one(1), 0);
> +
> +       /* Return add_one() to normal. */
> +       kunit_deactivate_static_stub(test, add_one);
> +       KUNIT_EXPECT_EQ(test, add_one(1), 2);
> +}
> +
>  /*
>   * Here we make a list of all the test cases we want to add to the test suite
>   * below.
> @@ -83,6 +121,7 @@ static struct kunit_case example_test_cases[] = {
>         KUNIT_CASE(example_simple_test),
>         KUNIT_CASE(example_skip_test),
>         KUNIT_CASE(example_mark_skipped_test),
> +       KUNIT_CASE(example_static_stub_test),
>         {}
>  };
>
> diff --git a/lib/kunit/static_stub.c b/lib/kunit/static_stub.c
> new file mode 100644
> index 000000000000..b06b88f82dd7
> --- /dev/null
> +++ b/lib/kunit/static_stub.c
> @@ -0,0 +1,125 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KUnit function redirection (static stubbing) API.
> + *
> + * Copyright (C) 2022, Google LLC.
> + * Author: David Gow <[e-mail]>
> + */
> +
> +#include <kunit/test.h>
> +#include <kunit/static_stub.h>
> +
> +
> +/* Context for a static stub. This is stored in the resource data. */
> +struct kunit_static_stub_ctx {
> +       void *real_fn_addr;
> +       void *replacement_addr;
> +};
> +
> +static void __kunit_static_stub_resource_free(struct kunit_resource *res)
> +{
> +       kfree(res->data);
> +}
> +
> +/* Matching function for kunit_find_resource(). match_data is real_fn_addr. */
> +static bool __kunit_static_stub_resource_match(struct kunit *test,
> +                                               struct kunit_resource *res,
> +                                               void *match_real_fn_addr)
> +{
> +       /* This pointer is only valid if res is a static stub resource. */
> +       struct kunit_static_stub_ctx *ctx = res->data;
> +
> +       /* Make sure the resource is a static stub resource. */
> +       if (res->free != &__kunit_static_stub_resource_free)
> +               return false;
> +
> +       return ctx->real_fn_addr == match_real_fn_addr;
> +}
> +
> +/* Returns the address of the replacement function. */
> +void *__kunit_get_static_stub_address(struct kunit *test, void *real_fn_addr)
> +{
> +       struct kunit_resource *res;
> +       struct kunit_static_stub_ctx *ctx;
> +       void *replacement_addr;
> +
> +       res = kunit_find_resource(test,
> +                                 __kunit_static_stub_resource_match,
> +                                 real_fn_addr);
> +
> +       if (!res)
> +               return NULL;
> +
> +       ctx = res->data;
> +       replacement_addr = ctx->replacement_addr;
> +       kunit_put_resource(res);
> +       return replacement_addr;
> +}
> +EXPORT_SYMBOL_GPL(__kunit_get_static_stub_address);
> +
> +void kunit_deactivate_static_stub(struct kunit *test, void *real_fn_addr)
> +{
> +       struct kunit_resource *res;
> +
> +       KUNIT_ASSERT_PTR_NE_MSG(test, real_fn_addr, NULL,
> +                               "Tried to deactivate a NULL stub.");
> +
> +       /* Look up the existing stub for this function. */
> +       res = kunit_find_resource(test,
> +                                 __kunit_static_stub_resource_match,
> +                                 real_fn_addr);
> +
> +       /* Error out if the stub doesn't exist. */
> +       KUNIT_ASSERT_PTR_NE_MSG(test, res, NULL,
> +                               "Tried to deactivate a nonexistent stub.");
> +
> +       /* Free the stub. We 'put' twice, as we got a reference
> +        * from kunit_find_resource()
> +        */
> +       kunit_remove_resource(test, res);
> +       kunit_put_resource(res);
> +}
> +EXPORT_SYMBOL_GPL(kunit_deactivate_static_stub);
> +
> +/* Helper function for kunit_activate_static_stub(). The macro does
> + * typechecking, so use it instead.
> + */
> +void __kunit_activate_static_stub(struct kunit *test,
> +                                 void *real_fn_addr,
> +                                 void *replacement_addr)
> +{
> +       struct kunit_static_stub_ctx *ctx;
> +       struct kunit_resource *res;
> +
> +       KUNIT_ASSERT_PTR_NE_MSG(test, real_fn_addr, NULL,
> +                               "Tried to activate a stub for function NULL");
> +
> +       /* If the replacement address is NULL, deactivate the stub. */
> +       if (!replacement_addr) {
> +               kunit_deactivate_static_stub(test, replacement_addr);
> +               return;
> +       }
> +
> +       /* Look up any existing stubs for this function, and replace them. */
> +       res = kunit_find_resource(test,
> +                                 __kunit_static_stub_resource_match,
> +                                 real_fn_addr);
> +       if (res) {
> +               ctx = res->data;
> +               ctx->replacement_addr = replacement_addr;
> +
> +               /* We got an extra reference from find_resource(), so put it. */
> +               kunit_put_resource(res);
> +       } else {
> +               ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
> +               KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);
> +               ctx->real_fn_addr = real_fn_addr;
> +               ctx->replacement_addr = replacement_addr;
> +               res = kunit_alloc_resource(test, NULL,
> +                                    &__kunit_static_stub_resource_free,
> +                                    GFP_KERNEL, ctx);
> +       }
> +}
> +EXPORT_SYMBOL_GPL(__kunit_activate_static_stub);
> +
> +
> --
> 2.35.1.894.gb6a874cedc-goog
>

Steve Muckle April 15, 2022, 10:05 p.m. UTC | #2

On 4/4/22 13:16, Brendan Higgins wrote:
> On Thu, Mar 17, 2022 at 10:13 PM David Gow <[e-mail]> wrote:
>> +/**
>> + * KUNIT_STATIC_STUB_REDIRECT() - call a replacement 'static stub' if one exists
>> + * @real_fn_name: The name of this function (as an identifier, not a string)
>> + * @args: All of the arguments passed to this function
>> + *
>> + * This is a function prologue which is used to allow calls to the current
>> + * function to be redirected by a KUnit test. KUnit tests can call
>> + * kunit_activate_static_stub() to pass a replacement function in. The
>> + * replacement function will be called by KUNIT_TRIGGER_STATIC_STUB(), which
>> + * will then return from the function. If the caller is not in a KUnit context,
>> + * the function will continue execution as normal.
>> + *
>> + * Example:
>> + *
>> + * .. code-block:: c
>> + *     int real_func(int n)
>> + *     {
>> + *             KUNIT_STATIC_STUB_REDIRECT(real_func, n);

I wish there was a way to avoid having to repeat the function's name and 
arguments here...

Daniel Latypov May 4, 2022, 8:35 p.m. UTC | #3

On Thu, Mar 17, 2022 at 9:13 PM David Gow <[e-mail]> wrote:
> +#define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {  \
> +       typecheck(typeof(real_fn_addr), replacement_addr);                      \

We can't call this macro in the same scope for functions w/ different
signatures.

E.g. if we add this func to the example test
  static void other_func(void) {}
then trying to call kunit_activate_static_stub() on it in the same
test case, we get

./include/linux/typecheck.h:10:14: error: conflicting types for
‘__dummy’; have ‘void(void)’
   10 | ({      type __dummy; \
      |              ^~~~~~~
./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
   99 |         typecheck(typeof(real_fn_addr), replacement_addr);
                 \
      |         ^~~~~~~~~
lib/kunit/example-test.c:64:9: note: in expansion of macro
‘kunit_activate_static_stub’
   64 |         kunit_activate_static_stub(test, other_func, other_func);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/typecheck.h:10:14: note: previous declaration of
‘__dummy’ with type ‘int(int)’
   10 | ({      type __dummy; \
      |              ^~~~~~~
./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
   99 |         typecheck(typeof(real_fn_addr), replacement_addr);
                 \
      |         ^~~~~~~~~
lib/kunit/example-test.c:62:9: note: in expansion of macro
‘kunit_activate_static_stub’
   62 |         kunit_activate_static_stub(test, add_one, subtract_one);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~

Afaict, the problem is that GCC thinks we're declaring a *function*
called __dummy, not a variable.
So it bleeds across the scope boundary of do-while unlike normal variables.

There's the typecheck_fn macro, but it doesn't work either.

Brendan Higgins May 4, 2022, 8:42 p.m. UTC | #4

On Wed, May 4, 2022 at 4:35 PM Daniel Latypov <[e-mail]> wrote:
>
> On Thu, Mar 17, 2022 at 9:13 PM David Gow <[e-mail]> wrote:
> > +#define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {  \
> > +       typecheck(typeof(real_fn_addr), replacement_addr);                      \
>
> We can't call this macro in the same scope for functions w/ different
> signatures.
>
> E.g. if we add this func to the example test
>   static void other_func(void) {}
> then trying to call kunit_activate_static_stub() on it in the same
> test case, we get
>
> ./include/linux/typecheck.h:10:14: error: conflicting types for
> ‘__dummy’; have ‘void(void)’
>    10 | ({      type __dummy; \
>       |              ^~~~~~~
> ./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
>    99 |         typecheck(typeof(real_fn_addr), replacement_addr);
>                  \
>       |         ^~~~~~~~~
> lib/kunit/example-test.c:64:9: note: in expansion of macro
> ‘kunit_activate_static_stub’
>    64 |         kunit_activate_static_stub(test, other_func, other_func);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/typecheck.h:10:14: note: previous declaration of
> ‘__dummy’ with type ‘int(int)’
>    10 | ({      type __dummy; \
>       |              ^~~~~~~
> ./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
>    99 |         typecheck(typeof(real_fn_addr), replacement_addr);
>                  \
>       |         ^~~~~~~~~
> lib/kunit/example-test.c:62:9: note: in expansion of macro
> ‘kunit_activate_static_stub’
>    62 |         kunit_activate_static_stub(test, add_one, subtract_one);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Afaict, the problem is that GCC thinks we're declaring a *function*
> called __dummy, not a variable.
> So it bleeds across the scope boundary of do-while unlike normal variables.

Yeah, I ran into that problem too. I posted a fix to gerrit. I have
been meaning to share it here.

> There's the typecheck_fn macro, but it doesn't work either.

That's weird. It worked for me.

Daniel Latypov May 4, 2022, 9:16 p.m. UTC | #5

On Wed, May 4, 2022 at 3:42 PM 'Brendan Higgins' via KUnit Development
<[e-mail]> wrote:
>
> On Wed, May 4, 2022 at 4:35 PM Daniel Latypov <[e-mail]> wrote:
> >
> > On Thu, Mar 17, 2022 at 9:13 PM David Gow <[e-mail]> wrote:
> > > +#define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {  \
> > > +       typecheck(typeof(real_fn_addr), replacement_addr);                      \
> >
> > We can't call this macro in the same scope for functions w/ different
> > signatures.
> >
> > E.g. if we add this func to the example test
> >   static void other_func(void) {}
> > then trying to call kunit_activate_static_stub() on it in the same
> > test case, we get
> >
> > ./include/linux/typecheck.h:10:14: error: conflicting types for
> > ‘__dummy’; have ‘void(void)’
> >    10 | ({      type __dummy; \
> >       |              ^~~~~~~
> > ./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
> >    99 |         typecheck(typeof(real_fn_addr), replacement_addr);
> >                  \
> >       |         ^~~~~~~~~
> > lib/kunit/example-test.c:64:9: note: in expansion of macro
> > ‘kunit_activate_static_stub’
> >    64 |         kunit_activate_static_stub(test, other_func, other_func);
> >       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
> > ./include/linux/typecheck.h:10:14: note: previous declaration of
> > ‘__dummy’ with type ‘int(int)’
> >    10 | ({      type __dummy; \
> >       |              ^~~~~~~
> > ./include/kunit/static_stub.h:99:9: note: in expansion of macro ‘typecheck’
> >    99 |         typecheck(typeof(real_fn_addr), replacement_addr);
> >                  \
> >       |         ^~~~~~~~~
> > lib/kunit/example-test.c:62:9: note: in expansion of macro
> > ‘kunit_activate_static_stub’
> >    62 |         kunit_activate_static_stub(test, add_one, subtract_one);
> >       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Afaict, the problem is that GCC thinks we're declaring a *function*
> > called __dummy, not a variable.
> > So it bleeds across the scope boundary of do-while unlike normal variables.
>
> Yeah, I ran into that problem too. I posted a fix to gerrit. I have
> been meaning to share it here.

For others, gerrit == https://kunit-review.googlesource.com/c/linux/+/5129

>
> > There's the typecheck_fn macro, but it doesn't work either.
>
> That's weird. It worked for me.

I'm running on top of 5.5.
I tried reproducing w/ a stripped down version on 5.18 and saw the same issues.

Huh, I'm trying with
 #define kunit_activate_static_stub(test, real_fn_addr,
replacement_addr) do {  \
-       typecheck(typeof(real_fn_addr), replacement_addr);
         \
+       typecheck_fn(typeof(real_fn_addr), replacement_addr); \
        __kunit_activate_static_stub(test, real_fn_addr,
replacement_addr);     \

This gives me
lib/kunit/example-test.c:62:9: error: function ‘__tmp’ is initialized
like a variable
   62 |         kunit_activate_static_stub(test, add_one, subtract_one);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
lib/kunit/example-test.c:64:9: error: function ‘__tmp’ is initialized
like a variable
   64 |         kunit_activate_static_stub(test, other_func, other_func);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~

Perhaps I'm missing something silly.

Can you post your fix and I can try it out?

Patch

diff --git a/include/kunit/static_stub.h b/include/kunit/static_stub.h
new file mode 100644
index 000000000000..020e0e9110f2
--- /dev/null
+++ b/include/kunit/static_stub.h
@@ -0,0 +1,106 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KUnit function redirection (static stubbing) API.
+ *
+ * Copyright (C) 2022, Google LLC.
+ * Author: David Gow <[e-mail]>
+ */
+#ifndef _KUNIT_STATIC_STUB_H
+#define _KUNIT_STATIC_STUB_H
+
+#if !IS_REACHABLE(CONFIG_KUNIT)
+
+/* If CONFIG_KUNIT is not enabled, these stubs quietly disappear. */
+#define KUNIT_TRIGGER_STATIC_STUB(real_fn_name, args...) do {} while (0)
+
+#else
+
+#include <kunit/test.h>
+
+#include <linux/compiler.h> /* for {un,}likely() */
+#include <linux/sched.h> /* for task_struct */
+
+/* Returns the address of the replacement function. */
+void *__kunit_get_static_stub_address(struct kunit *test, void *real_fn_addr);
+
+/**
+ * KUNIT_STATIC_STUB_REDIRECT() - call a replacement 'static stub' if one exists
+ * @real_fn_name: The name of this function (as an identifier, not a string)
+ * @args: All of the arguments passed to this function
+ *
+ * This is a function prologue which is used to allow calls to the current
+ * function to be redirected by a KUnit test. KUnit tests can call
+ * kunit_activate_static_stub() to pass a replacement function in. The
+ * replacement function will be called by KUNIT_TRIGGER_STATIC_STUB(), which
+ * will then return from the function. If the caller is not in a KUnit context,
+ * the function will continue execution as normal.
+ *
+ * Example:
+ *
+ * .. code-block:: c
+ *	int real_func(int n)
+ *	{
+ *		KUNIT_STATIC_STUB_REDIRECT(real_func, n);
+ *		return 0;
+ *	}
+ *
+ *	void replacement_func(int n)
+ *	{
+ *		return 42;
+ *	}
+ *
+ *	void example_test(struct kunit *test)
+ *	{
+ *		kunit_activate_static_stub(test, real_func, replacement_func);
+ *		KUNIT_EXPECT_EQ(test, real_func(1), 42);
+ *	}
+ *
+ */
+#define KUNIT_STATIC_STUB_REDIRECT(real_fn_name, args...)		\
+do {									\
+	typeof(&real_fn_name) replacement;				\
+	struct kunit *current_test = current->kunit_test;		\
+									\
+	if (likely(!current_test))					\
+		break;							\
+									\
+	KUNIT_ASSERT_STREQ_MSG(current_test, __func__, #real_fn_name,	\
+			       "Static stub function name mismatch");	\
+									\
+	replacement = __kunit_get_static_stub_address(current_test,	\
+							&real_fn_name);	\
+									\
+	if (unlikely(replacement))					\
+		return replacement(args);				\
+} while (0)
+
+/* Helper function for kunit_activate_static_stub(). The macro does
+ * typechecking, so use it instead.
+ */
+void __kunit_activate_static_stub(struct kunit *test,
+				  void *real_fn_addr,
+				  void *replacement_addr);
+
+/**
+ * kunit_activate_static_stub() - replace a function using static stubs.
+ * @test: A pointer to the 'struct kunit' test context for the current test.
+ * @real_fn_addr: The address of the function to replace.
+ * @replacement_addr: The address of the function to replace it with.
+ *
+ * When activated, calls to real_fn_addr from within this test (even if called
+ * indirectly) will instead call replacement_addr. The function pointed to by
+ * real_fn_addr must begin with the static stub prologue in
+ * KUNIT_TRIGGER_STATIC_STUB() for this to work. real_fn_addr and
+ * replacement_addr must have the same type.
+ *
+ * The redirection can be disabled again with kunit_deactivate_static_stub().
+ */
+#define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {	\
+	typecheck(typeof(real_fn_addr), replacement_addr);			\
+	__kunit_activate_static_stub(test, real_fn_addr, replacement_addr);	\
+} while (0)
+
+void kunit_deactivate_static_stub(struct kunit *test, void *real_fn_addr);
+
+#endif
+#endif
diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index c49f4ffb6273..f9e929700782 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -1,6 +1,7 @@ 
 obj-$(CONFIG_KUNIT) +=			kunit.o
 
 kunit-objs +=				test.o \
+					static_stub.o \
 					string-stream.o \
 					assert.o \
 					try-catch.o \
diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c
index 51099b0ca29c..670c21e74446 100644
--- a/lib/kunit/kunit-example-test.c
+++ b/lib/kunit/kunit-example-test.c
@@ -7,6 +7,7 @@ 
  */
 
 #include <kunit/test.h>
+#include <kunit/static_stub.h>
 
 /*
  * This is the most fundamental element of KUnit, the test case. A test case
@@ -69,6 +70,43 @@  static void example_mark_skipped_test(struct kunit *test)
 	/* This line should run */
 	kunit_info(test, "You should see this line.");
 }
+
+/* This is a function we'll replace with static stubs. */
+static int add_one(int i)
+{
+	/* This will trigger the stub if active. */
+	KUNIT_STATIC_STUB_REDIRECT(add_one, i);
+
+	return i + 1;
+}
+
+/* This is used as a replacement for the above function. */
+static int subtract_one(int i)
+{
+	/* We don't need to trigger the stub from the replacement. */
+
+	return i - 1;
+}
+
+/*
+ * This test shows the use of static stubs.
+ */
+static void example_static_stub_test(struct kunit *test)
+{
+	/* By default, function is not stubbed. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+
+	/* Replace add_one() with subtract_one(). */
+	kunit_activate_static_stub(test, add_one, subtract_one);
+
+	/* add_one() is now replaced. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 0);
+
+	/* Return add_one() to normal. */
+	kunit_deactivate_static_stub(test, add_one);
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+}
+
 /*
  * Here we make a list of all the test cases we want to add to the test suite
  * below.
@@ -83,6 +121,7 @@  static struct kunit_case example_test_cases[] = {
 	KUNIT_CASE(example_simple_test),
 	KUNIT_CASE(example_skip_test),
 	KUNIT_CASE(example_mark_skipped_test),
+	KUNIT_CASE(example_static_stub_test),
 	{}
 };
 
diff --git a/lib/kunit/static_stub.c b/lib/kunit/static_stub.c
new file mode 100644
index 000000000000..b06b88f82dd7
--- /dev/null
+++ b/lib/kunit/static_stub.c
@@ -0,0 +1,125 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit function redirection (static stubbing) API.
+ *
+ * Copyright (C) 2022, Google LLC.
+ * Author: David Gow <[e-mail]>
+ */
+
+#include <kunit/test.h>
+#include <kunit/static_stub.h>
+
+
+/* Context for a static stub. This is stored in the resource data. */
+struct kunit_static_stub_ctx {
+	void *real_fn_addr;
+	void *replacement_addr;
+};
+
+static void __kunit_static_stub_resource_free(struct kunit_resource *res)
+{
+	kfree(res->data);
+}
+
+/* Matching function for kunit_find_resource(). match_data is real_fn_addr. */
+static bool __kunit_static_stub_resource_match(struct kunit *test,
+						struct kunit_resource *res,
+						void *match_real_fn_addr)
+{
+	/* This pointer is only valid if res is a static stub resource. */
+	struct kunit_static_stub_ctx *ctx = res->data;
+
+	/* Make sure the resource is a static stub resource. */
+	if (res->free != &__kunit_static_stub_resource_free)
+		return false;
+
+	return ctx->real_fn_addr == match_real_fn_addr;
+}
+
+/* Returns the address of the replacement function. */
+void *__kunit_get_static_stub_address(struct kunit *test, void *real_fn_addr)
+{
+	struct kunit_resource *res;
+	struct kunit_static_stub_ctx *ctx;
+	void *replacement_addr;
+
+	res = kunit_find_resource(test,
+				  __kunit_static_stub_resource_match,
+				  real_fn_addr);
+
+	if (!res)
+		return NULL;
+
+	ctx = res->data;
+	replacement_addr = ctx->replacement_addr;
+	kunit_put_resource(res);
+	return replacement_addr;
+}
+EXPORT_SYMBOL_GPL(__kunit_get_static_stub_address);
+
+void kunit_deactivate_static_stub(struct kunit *test, void *real_fn_addr)
+{
+	struct kunit_resource *res;
+
+	KUNIT_ASSERT_PTR_NE_MSG(test, real_fn_addr, NULL,
+				"Tried to deactivate a NULL stub.");
+
+	/* Look up the existing stub for this function. */
+	res = kunit_find_resource(test,
+				  __kunit_static_stub_resource_match,
+				  real_fn_addr);
+
+	/* Error out if the stub doesn't exist. */
+	KUNIT_ASSERT_PTR_NE_MSG(test, res, NULL,
+				"Tried to deactivate a nonexistent stub.");
+
+	/* Free the stub. We 'put' twice, as we got a reference
+	 * from kunit_find_resource()
+	 */
+	kunit_remove_resource(test, res);
+	kunit_put_resource(res);
+}
+EXPORT_SYMBOL_GPL(kunit_deactivate_static_stub);
+
+/* Helper function for kunit_activate_static_stub(). The macro does
+ * typechecking, so use it instead.
+ */
+void __kunit_activate_static_stub(struct kunit *test,
+				  void *real_fn_addr,
+				  void *replacement_addr)
+{
+	struct kunit_static_stub_ctx *ctx;
+	struct kunit_resource *res;
+
+	KUNIT_ASSERT_PTR_NE_MSG(test, real_fn_addr, NULL,
+				"Tried to activate a stub for function NULL");
+
+	/* If the replacement address is NULL, deactivate the stub. */
+	if (!replacement_addr) {
+		kunit_deactivate_static_stub(test, replacement_addr);
+		return;
+	}
+
+	/* Look up any existing stubs for this function, and replace them. */
+	res = kunit_find_resource(test,
+				  __kunit_static_stub_resource_match,
+				  real_fn_addr);
+	if (res) {
+		ctx = res->data;
+		ctx->replacement_addr = replacement_addr;
+
+		/* We got an extra reference from find_resource(), so put it. */
+		kunit_put_resource(res);
+	} else {
+		ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);
+		ctx->real_fn_addr = real_fn_addr;
+		ctx->replacement_addr = replacement_addr;
+		res = kunit_alloc_resource(test, NULL,
+				     &__kunit_static_stub_resource_free,
+				     GFP_KERNEL, ctx);
+	}
+}
+EXPORT_SYMBOL_GPL(__kunit_activate_static_stub);
+
+